The invention relates to the field of waveguide fabrication, and in particular to fabricating high index difference waveguides using a high index contrast grating.
High index difference waveguides provide for very small optical devices, thus allowing for dense integration of optical components. One of the difficulties with high index contrast waveguides is their small mode field diameter. As a result, coupling of light from a fiber to a device with a mode diameter of 1 μm or less is difficult.
Gratings have often been used to change the direction of light propagation. Most gratings are low index gratings that have a waveguide effective index difference between the high and low regions of less than 0.3. Applications of low index gratings include coupling of light in and out of waveguides, especially slab waveguides, and coupling light from waveguides or detectors.
High index difference gratings have been used in optical micro-cavities devices, but have not been explored for use in coupling modes in and out of high index difference waveguides. The reason is that low index gratings have much lower scattering loss resulting in higher overall efficiency.